Syndepositional and diagenetic processes in the pigmentation of Middle Ordovician carbonate red beds in South China

Abstract

Marine red beds (MRBs) are often attributed to specific redox environments during syndepositional and early diagenetic phases. During the Middle Ordovician, a succession of reddish, deeper-water nodular argillaceous limestones (i.e., Zitai and Kuniutan formations) were deposited along the margin of Yangtze Platform in South China. However, the origin of their color remains enigmatic. In this study, we investigated the Middle Ordovician MRBs from a borehole core newly drilled in the Lower Yangtze area of South China whose stratigraphy frameworks are constrained by carbon isotope and biostratigraphy. This study investigates the pigmentation of these MRBs by integrating petrographic observations, elemental geochemistry, diffusive reflectance spectrometry (DRS), and scanning electron microscope (SEM) coupled with energy dispersive X-ray spectrometry (EDS). DRS results show that the red pigment is caused by hematite particles in submicron- to micron-level size. SEM demonstrates that the hematite grains are either detrital grains with traces of physical transport from terrestrial source, or flaky amorphous hematite aggregates situated within the calcite crystal interstices, implicating both syndepositional and early diagenetic origins, respectively. In terms of the geochemical result of the bulk rocks, the close positive correlation between Al₂O₃ and Fe₂O₃ indicates that the iron pigment materials may mainly originate from terrestrial Fe-bearing phyllosilicates. These observations are also consistent with the distribution patterns of rare earth elements (REEs) in carbonate leachates. The MRB limestones are characterized by MREE-bulged patterns and close to ~1 Ce anomalies, suggesting active reductive dissolution and subsequent reprecipitation of iron oxides during diagenesis in pore systems. This study proposes that the coloration of Middle Ordovician MRBs in Lower Yangtze Platform was linked to the enhanced input of terrestrial clay minerals rich in iron. The reductive dissolution released iron ions from terrestrial detrital and allowed subsequent reprecipitation of iron-oxides in pore water system during the fluid-buffered diagenesis. In this light, hematites formed during both the syndepositional and diagenetic processes thus could have involved the coloration of the Middle Ordovician carbonate red beds in this case.